This invention relates generally to the field of reflow soldering ovens, and, more particularly to a method and an apparatus for removing flux vapors and other volatile contaminants from the atmosphere of such ovens.
Printed circuit boards are commonly fabricated using the reflow solder technique. A paste containing solder particles mixed with flux, adhesives, binders, and other components is applied to selected areas of a printed circuit board. Electronic devices are pressed into the applied solder paste. Adhesives in the paste hold the devices to the printed circuit board. A conveyor within a reflow oven tunnel carries the printed circuit board and devices through regions of varying temperatures within the oven, each region performing a specific step in the reflow process. Generally an initial (preheat) region will bring the circuit board and devices to a predetermined temperature for flux reaction. A second (soak) region will maintain this reaction temperature for a predetermined time. Flux in the solder paste reacts with the contacts to remove oxides and to enhance wetting. A third (reflow) region will heat the circuit boards and devices to a temperature sufficient to cause the solder particles in the paste to melt. Molten solder wets metal contacts on the devices and printed circuit board. The conveyor moves the heated printed circuit board to a fourth (cooling) region of the oven where the molten solder solidifies forming a completed electronic circuit.
The reaction of the flux with the contacts liberates vapors. Further, heat within the oven vaporizes flux as well as adhesives, binders, and other components of the solder paste, devices, and circuit boards. The vapors from these materials accumulate within the oven leading to a number of problems. If the vapors migrate to a cooler region they will condense on the circuit boards, contaminating the boards and making subsequent cleaning steps necessary. The vapors will also condense on cooler surfaces within the oven, clogging gas orifices, gumming up moving parts, and creating a fire hazard. This condensate may also drip onto subsequent circuit boards destroying them, or making subsequent cleaning steps necessary. In addition, condensed vapors may contain corrosive and toxic chemicals, which can damage equipment and create a hazard to personnel.
The flux vapors and other volatile compounds generated by the reflow operation collectively are referred to in this application as xe2x80x9cvolatile contaminants.xe2x80x9d It is understood that this term is intended to encompass all reaction products released when the printed circuit board is heated, including flux vapors, vapors from all other components of the solder paste, as well as all vapors out-gassed from the printed circuit board and the electronic devices.
Volatile contaminants can be flushed from the oven by providing a continuous supply of clean gas. This is commonly done in machine operating with air as the process gas by drawing fresh air in both ends of the oven and exhausting the air along with volatile contaminants from the inner regions of the oven as described in U.S. Pat. No. 5,345,061 by Chanasyk et al. This is not an ideal solution for the cases where the oven must be filled with a substantially inert gas, for example nitrogen. Generating additional inert gas in volumes adequate to flush these ovens (2000 cfh or more) is expensive.
The approaches of volatile contaminant removal that do not involve condensing tend to lack feasibility due to cost, scientific complexity, physical impracticality or a combination of all of these. Incineration, for instance, requires impracticably high temperatures. UV decomposition utilized very expensive equipment. Filtration in the gaseous state requires a filter far to fine to consider at the volumes needed. Nuclear disintegration has obvious problems related to radioactive materials.
Generally, the prior art involves removal, filtration, and reintroduction of inert gas to the oven, however, they fail to address the complex gas flow patterns necessary to maintain a clean, substantially inert atmosphere while also maintaining the precise temperature profile required by the reflow process.
These complex flow patterns in the tunnel are both natural and created. They may either help or hinder attempts to optimize various process parameters. It is necessary to consider each of these various parameters separately before designing a flow scheme that is the best compromise for the process as a whole.
Flows for substantially inert gas optimization need to purge the entire tunnel of air and pressurize the center such that flows are out both ends impeding any flow of outside environment in. Flows optimized for substantially inert gas containment are not compatible with control of volatile contaminant condensation as all volatile contaminants pass through the coolest regions of the oven where they condense before they reach the exhaust, and also may not be optimized for zone temperature definition.
It is therefore an object of this invention to provide an apparatus and a method for the control of the complex gas flow patterns in a reflow soldering oven to maintain a clean inert atmosphere while maintaining the precise temperature profile.
It is another object of this invention to provide an apparatus and a method to reduce operating expenses by conserving substantially inert gas.
It is yet another objective of this invention to provide an apparatus and a method to minimize the sensitivity of oven external pressure and/or temperature variations on the oven internal environment.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described herein below.
The present invention introduces an substantially inert gas, for example nitrogen, in a reflow oven tunnel at predetermined points to influence the oven tunnel gas flow and to dilute and expel excess oxygen from the system. Tunnel gas is ducted from the tunnel at selected points to a flux separation system to be cleaned, for example remove flux vapors, and returned to the tunnel for recirculation, thereby conserving the substantially inert gas and maintaining a low oxygen environment. Additionally, the substantially inert gas is introduced in amounts larger than required to create and maintain tunnel gas flow. The system maintains sufficient pressure to supply end baffle boxes with clean gas to create a gas barrier to effectively seal off the tunnel ends from air infiltration or migration into the tunnel making the system insensitive to external pressure and/or temperature variations.
The substantially inert gas, such as nitrogen, is cold relative to the tunnel gas at the point of introduction. The cold gas will mix with the hotter gas thereby absorbing excess heat for better zone temperature control. The cold source gas mixing with the hotter gas causes an expansion of the tunnel gas. The tunnel gas is drawn in both directions by the tunnel gas outlets of the recirculation system.
Preferably, substantially inert gas is introduced into the pre-heat and/or soak zones of the tunnel mixing with the tunnel gas. The mixed gas primarily flows toward the reflow zone to prevent condensation and for zone temperature definition. Some mixed gas splits and flows toward select outlets in the pre-heat and soak zones to be ducted to the flux separation system for cleaning prior to recirculation. Additional inert gas is introduced into the cooling zone of the tunnel mixing with the tunnel gas and flowing back toward the reflow zone. The counter flowing gases converge at the boundary of the high temperature region and a cooling region to mix and exit the tunnel toward the flux separation system for cleaning and recirculation. The amount of substantially inert gas can be varied at each introduction inlet to account for gas expansion differentials and flow direction.
After cleaning, some of the clean gas is ducted back to recirculation ports for introduction back into the tunnel and the remaining clean gas is ducted to the end baffle boxes located at either or both ends of the oven tunnel to create a gas barrier preventing outside air from entering the tunnel. Recirculated gas can be reintroduced at points in the tunnel to cause flow generally from colder areas into hotter areas, with maximum flow out of the cool zones and flow out of a zone where flux build up problems are the worst. Recirculated flows are introduced into the baffle boxes for pressure control, baffling, and exhaust. The system exhausts through the baffle boxes more gas than input to account for expansion of the substantially inert gas due to heating.
The exhaust system, defined by the baffle boxes, recirculated gas and exhaust hoods, generally are not be used to remove contamination from the system, but only to remove the excess expansion gas, which has been cleaned, recirculated, used to pressurize the baffle box, and allowed to draft out of the baffle space to the exhaust hood, thereby inhibiting the in-flow of oxygen contained in the outside air. The exhaust hoods are located above the tunnel ends outside of the baffle boxes. The exhaust hoods can be ducted to the customers facility exhaust. The flow from the recirculation system into the baffle boxes are preferably set at a flow equal to the substantially inert gas input plus expansion to keep the pressure inside the tunnel constant, with no-flow through the baffle area and into the oven tunnel.
Baffle boxes are, preferably, incorporated at both ends of the oven to form isolation zones between the constantly changing pressure and drafts of the factory and the expanding and contracting gas flows within the tunnel. Since the baffle box preferably is pressurized by clean recirculated gas at a rate slightly higher than the substantially inert gas, such as nitrogen, input rate plus an expansion factor, the recirculated gas is metered through a series of small orifices onto a flat angled surface to create a series of low-level laminar flows. These flows come from both top and bottom of the reflow oven. The outer most flows are, preferably, allowed to draft out of the oven to be picked up by a low level flow exhaust hoods at the ends of the tunnel constituting the entire exhaust volume. The innermost flows will cause a pressurized space to keep expansion gas from the tunnel from flowing through the baffle space. The fact that the baffle boxes are supplied with recirculated gas is significant in several ways. It is preferred that no inert gas is dumped to the factory essentially unused and un-expanded; thus decreasing inert gas consumption. There is also no tunnel gas migrating through the baffle area disturbing the desired flow. This action also de-couples the flow of the baffle boxes from the flow in the tunnel in order to reduce the buoyancy driven mixing at the tunnel ends. Also since this recirculated gas is heated and ducted to top and bottom of both tunnel ends, the buoyancy effect is much reduced.
In alternative embodiments, low oxygen content in the tunnel gas is desired to improve system performance, for example filtration of contaminants. In such cases, low flow valves are employed to bleed low volumes of air into the tunnel at one or more pre-selected locations, thereby controlling the parts per million of oxygen mixed with the tunnel gas.
In air only machines, the possibility exists to exhaust the cleaned gas up a stack rather than returning it to the oven. This may result in an extremely clean air machine with simple gas flow, with resultant possible energy conservation.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawing and detailed description of preferred embodiments of the invention.